Modification of primary afferent depolarization in cat group Ia afferents following high frequency intra-axonal tetanization of individual afferents

Intra-axonal tetanization of a single, functionally-identified group Ia afferent from the triceps surae muscle in the anesthetized cat produces marked enhancement and slowing of the primary afferent depolarization (PAD) generated in the Ia afferent by volleys in flexor muscle group Ia afferents, plus a pronounced transmembrane hyperpolarizing undershoot (HPU) which disappears more rapidly than the enhanced PAD. These alterations are qualitatively and quantitatively similar to those found after conditioning tetani are applied to the whole muscle nerve. The occurrence of these PAD changes after intra-axonal tetanization of a single group Ia afferent appears to rule out the participation of non-specific alterations in extracellular ionic concentrations or activation of polysynaptic pathways in their genesis.